In recent years, water absorbing agents containing water absorbing resins have been used as components of sanitary goods such as, paper diapers (disposable diapers), sanitary napkins, and so-called incontinence pads for a purpose of absorbing body fluids.
Examples of well-known water absorbing resins include a partially neutralized and crosslinked polyacrylic acid, a hydrolyzed starch-acrylic acid graft polymer, a saponified vinyl acetate-acrylate copolymer, a hydrolyzed acrylonitrile copolymer or acrylamide copolymer, a crosslinked acrylonitrile copolymer or acrylamide copolymer, and a crosslinked cationic monomer.
A desirable water absorbing resin, when brought into contact with an aqueous liquid such as a body fluid, should exhibit excellent absorbency, absorbing rate, liquid permeability, gel strength of gel swollen with liquid, and suction force to suck the liquid from a base material containing the liquid. However, these characteristics are not necessarily correlated positively to one another. For example, a material boasting good absorbency is inferior in properties such as the liquid permeability, gel strength and absorbing rate.
It is known that those water absorbing characteristics of the water absorbing resin can be improved in a well-balanced manner by crosslinking the water absorbing resin in a neighborhood of the surface thereof, and various methods have been disclosed so far.
Examples are: a method of using a polyhydric alcohol as a crosslinking agent, disclosed in Japanese Laid-Open Patent Applications No. 58-180233/1983 (Tokukaisho 58-180233) and No. 61-16903/1986 (Tokukaisho 61-16903); a method of using a polyglycidyl compound, a polyfunctional aziridine, a polyfunctional amine compound, and a polyfunctional isocyanate compound as a crosslinking agent, disclosed in Japanese Laid-Open Patent Application No. 59-189103/1984 (Tokukaisho 59-189103); a method of using a glyoxal as a crosslinking agent, disclosed in Japanese Laid-Open Patent Application No. 52-117393/1977 (Tokukaisho 52-117393); a method of using a polyvalent metal as a crosslinking agent, disclosed in Japanese Laid-Open Patent Applications No. 51-136588/1976 (Tokukaisho 51-136588), No. 61-257235/1986 (Tokukaisho 61-257235) and No. 62-7745/1987 (Tokukaisho 62-7745); a method of using a silane coupling agent as a crosslinking agent, disclosed in Japanese Laid-Open Patent Applications No. 61-211305/1986 (Tokukaisho 61-211305), No. 61-252212/1985 (Tokukaisho 61-252212) and No. 61-264006/1985 (Tokukaisho 61-264006); and a method of using an alkylene carbonate as a crosslinking agent, disclosed in German Patent No. DE 40 20 780CI. Other examples are: a method of carrying out crosslinking reaction in the presence of inert inorganic particles, disclosed in Japanese Laid-Open Patent Applications No. 60-163956/1985 (Tokukaisho 60-163956) and No. 60-255814/1985 (Tokukaisho 60-255814); a method of carrying out crosslinking reaction in the presence of a dihydric alcohol, disclosed in Japanese Laid-Open Patent Application No. 1-292004/1989 (Tokukaihei 1-292004); a method of carrying out crosslinking reaction in the presence of water and an ether compound, disclosed in Japanese Laid-Open Patent Application No. 2-153903/1990 (Tokukaihei 2-153903); and a method of carrying out crosslinking reaction in the presence of an addition product of alkylene oxide to a monohydric alcohol, an organic acid salt, lactam, or the like, disclosed in European Patent No. EP 0 555 692 AI.
Generally, the water absorbing resin preferably contains as few particles having a diameter of not more than 150 .mu.m (fine particles) as possible. An absorbent product such as a diaper can be clogged up by those fine particles, resulting in a lower liquid permeability. The fine particles can be wasted as particle dust and can be a cause for pneumoconiosis when handled. Besides, even when the surface is crosslinked, properties, including the absorbency under pressure, are hard to improve by fine particles. For these reasons, there is a strong demand for water absorbing resins with few fine particles.
Conventionally, water absorbing resins with few fine particles have been prepared with 1 a method of adjusting particle sizes by adjusting polymerization, grinding, etc. and 2 a method of removing the produced fine particles with, for example, a sieve and/or air flow as disclosed in U.S. Pat. No. 4,973,632.
However, the adoption of Method 1 still produces quite a large amount of fine particles as much as a dozen percent to several dozen percent in the manufacturing process. Therefore, further removing the produced fine particles with Method 2 brings down the yield and can be a cause for losing a competitive edge in cost.
Suggestions are made to solve the problems by granulating or reproducing the fine particles inevitably produced in the manufacturing process of the water absorbing resin, and to accomplish a high absorbing rate by increasing the surface areas of the primary particles with granulation.
There are a number of patents disclosing methods other than granulation: for instance, European Patent No. 0,463,388B, U.S. Pat. No. 4,950,692 and U.S. Pat. No. 4,970,267, and European Patents No. 0,417,761A and 0,496,594A disclose methods of reproducing larger particles from fine particles by mixing the fine particles with water or with a water-containing gel so that the fine particles gel, and then by grinding and drying the gelled product. European Patent No. 0,644,224 discloses a method of granulating the fine particles by adding a water solution containing a water-soluble or water-dispersing macromolecular compound to a water absorbing resin in the presence of insoluble inorganic fine particles so that the granulated substance contains 30 to 70 weight percent of water. U.S. Pat. No. 5,002,986, European Patent No. 0,318,989B, U.S. Pat. No. 5,248,709, U.S. Pat. No. 4,123,397 and U.S. Pat. No. 4,734,478, European Patent No. 0,629,411, and U.S. Pat. No. 5,369,148 disclose methods of increasing the average particle diameter of fine particles to several hundreds of .mu.m with granulation by using fine particles of about 150 .mu.m to several tens of .mu.m either alone or as a mixture with larger particles and by using a water solution and the like as a binder in a ratio of several percent to slightly more than 20% of the particles.
Generally, water and an aqueous liquid are the most preferable binder for a water absorbing resin, in terms of efficiency, safety, manufacturing cost, etc. For this reason, an aqueous liquid which behaves like a binder is added to the fine particles in most of the various methods listed above.
However, a water absorbing resin, especially a water absorbing resin in a fine particle form, has a large surface area, and therefore has a high absorbing rate and makes it difficult to add the aqueous liquid uniformly. The use of insoluble inorganic fine particles or the like as a mixture assistant for the aqueous liquid poses not only a problem of high costs, but also problems of producing particle dust and of degrading the granulation strength and various properties of the substance.
Hence, there are various patents disclosing mixers which are capable of uniformly adding water when granulating the fine particles: a low-speed paddle type mixer (European Patent No. 0,644,224), a high-speed agitation type mixer (U.S. Pat. No. 5,002,986 and U.S. Pat. No. 4,734,478), a particular spray continuous granulating apparatus (U.S. Pat. No. 5,360,148), and a fluidized bed (European Patent No. 05,342,899). Suggested methods other than the granulation method include mixers for recycling the fine particles such as a nauta mixer (U.S. Pat. No. 4,950,692) and a particular shearing mixer (European Patent No. 0,417,761). Among these mixers, the high-speed agitation type mixer is not only used for the granulation methods, but also widely used for other purposes in a manufacturing process of the water absorbing resin, e.g., for the above-mentioned crosslinking of the neighborhood of the surface (U.S. Pat. No. 5,140,076), because of its high productivity.
For instance, the inventors of the present invention have disclosed in U.S. Pat. No. 5,140,076 (Japanese Laid-Open Patent Application No. 4-214734/1992 (Tokukaihei 4-214734)) a method of manufacturing a water absorbing agent by efficiently mixing a water solution of a crosslinking agent, a processing liquid containing a crosslinking agent, or the like to a surface of a water absorbing resin. More specifically, the surface crosslinking agent is reacted by mixing a crosslinking agent with a water absorbing resin, using a high-speed agitation type mixer having an inner surface composed of a particular base material and then by heating the mixed substance. In addition, the inventors of the present invention have obtained a water absorbing resin with an improved balance among the water absorbing characteristics, especially, with better absorbency under pressure.
FIG. 21 shows a conventional mixer disclosed in U.S. Pat. No. 5,140,076. The mixer 100 is of a continuous extruding type and is provided with a plurality of agitating blades 103 around a rotational shaft 102 which is disposed in a fixed cylinder 101 having an inner surface composed of a particular base material. In the continuous extending mixer 100, particles of a water absorbing resin are supplied through a resin supply port 104, and a crosslinking agent is supplied through a liquid injection port 105. Then, the mixture is taken out of an ejection port 106 as it is extruded forward by the rotation of the agitating blades 103.
However, it has been found that even when water absorbing resin particles are crosslinked in a neighborhood of the surface thereof with the conventional continuous extruding type mixer, problems still arise with end products, such as diapers, that they lose the expected excellent water absorbing characteristics, possibly because the crosslinked or granulated parts fall off or are disintegrated when the water absorbing resin is transported or when the water absorbing resin is processed by a user into end products. It has been also found that the problem becomes more evident with a water absorbing resin boasting a high water absorbency under high pressure.
It has been also found that the granulation or reproduction of the fine particles in some cases degrades properties: for example, the absorbing rate decreases, the water-soluble components that are impurities increase, and the absorbency under high pressure decreases. It has been also found that the granulation degrades the properties more when a aqueous liquid as a binder is added in a larger amount to enhance the granulation strength and thus prevent the above mentioned disintegration of granulation.
A cause of the above problems is that the water absorbing resin is not satisfactorily mixed with the crosslinking agent, the aqueous liquid, etc., using the conventional mixer.
For example, the aqueous liquid is added in a ratio as low as several percent to 30% at most when the fluidized bed (European Patent No. 05,342,899) or the high-speed agitation type mixer (U.S. Pat. No. 5,140,076) is used for granulating the fine particles. If the aqueous liquid is added in a ratio exceeding 60%, it is extremely difficult to mix stably and continuously.
Moreover, the inventors of the present invention have found that if the aqueous liquid is added in a ratio exceeding 10%, the conventional mixer can only mix the water absorbing resin and the aqueous liquid in an extremely non-uniform manner. As a result, the aqueous liquid for enhancing the granulation strength can be added only in a limited amount, and the non-uniform addition of the aqueous liquid even degrades the properties and causes granulation to be disintegrated. Especially, with the high-speed agitation type mixer conventionally used for manufacture of a water absorbing resin, a small amount of aqueous liquid results in a high productivity, but it is almost impossible to add a aqueous liquid in a large amount.
Moreover, the inventors of the present invention have found that although the aqueous liquid can be added relatively easily with mixers having a high kneading capability such as the shearing mixer (European Patent No. 0,417,761) and the nauta mixer having been used for other purposes than granulation, the mixture obtained after the addition of a aqueous liquid, not being granulated in a particle form, is a huge mass of gel. Besides, the shearing force degrades the water absorbing resin. Furthermore, if the water-containing gel of the water absorbing resin is kneaded or ground with too large a force, the water-containing gel, not becoming an agglomerate, but becomes gel of a small surface area that seems to have been kneaded and crushed. Therefore, in some cases, even if fine particles of large surface areas are used as raw material, the absorbing rate decreases on the contrary. The inventors of the present invention have also found that a process to grind the huge mass of gel produced with a method other than granulation is likely to degrade the water absorbing resin.
In view of the problems, objects of the present invention is to offer a water absorbing agent, a manufacturing method thereof, and a manufacturing machine thereof, the water absorbing agent being capable of exhibiting high absorbency under pressure, of maintaining its high water absorbing performance even after, for example, being transported in a manufacturing plant or being processed into end products by a user, of being used in any manner in end products, and of always exhibiting excellent characteristics.